Current transformer assembly

ABSTRACT

A current transformer assembly includes a shunt circuit connected to a main circuit, the shunt circuit having n number of shunt conductors; a main current transformer for transforming current passing through one of the shunt conductors; and (n-1) number of current balancers having secondary windings connected to each other to enable a circulating secondary current flow. Each of the current balancers is formed of two subcurrent transformers so as to fix the ratio of the magnitude of the currents conducted through the individual shunt conductors.

United States Patent [151 3,665,357 [451 May 23,1972

Tsubouchi et a1.

CURRENT TRANSFORMER ASSEMBLY Denji Tsubouchi, Yokahama; Kenjl Wada, Yamato, both of Japan Tokyo Shibaura Electric Company, Ltd., Kawasaki-shi, Japan Mar. 15, 1971 Inventors:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Mar. 16, 1970 Japan ..45/2l542 Mar. 28, 1970 Japan ..45/25821 Mar. 19, 1970 Japan ..45/22881 US. Cl ..336/73, 336/150, 336/174, 336/175 Int. Cl. ..H0lf 40/06 Field of Search ..336/173, 174, 175, 150, 73, 336/180, 170, 94

References Cited UNITED STATES PATENTS Hauch ..336/175 1 FOREIGN PATENTS OR APPLICATIONS 1,405,373 5/1965 France ..336/173 1,165,741 3/1964 Germany ..336/173 1,254,239 11/1967 Germany ..336/173 Primary Examiner-Thomas J. Kozma Attorney-Oblon, Fisher & Spivak ABSTRACT A current transformer assembly includes a shunt circuit connected to a main circuit the shunt circuit having n number of shunt conductors; a main current transformer for transforming current passing through one of the shunt conductors; and

(n- 1) number of current balancers having secondary windings connected to each other to enable a circulating secondary current flow. Each of the current balancers is formed of two subcurrent transformers so as to fix the ratio of the magnitude of the currents conducted through the individual shunt conductors.

5 Claims, 9 Drawing figures Patented May 23, 1972 3 Sheets-Sheet 1 FIG. 2B

INVENTORS DENJI TSUBOUCHI and KENJI WADA BY (966M, #34 3 ATTORNEYS Patented May 23, 1972 3 Sheets-Sheet 2 FIG. 1*

CURRENT TRANSFORMER ASSEMBLY BACKGROUND OF THE INVENTION windings and an iron core, and is designed so as to transform the primary current through the core and derive out of the secondary winding a current stepped down in proportion to that conducted through the primary winding. Accordingly, it is necessary that the secondary winding-current always be exactly proportionate to the primary winding current. However, the conventional current transformer device is of the type where all of the rated current passes through the primary winding and consequently has the drawback thatthe fonn of the current transformer device is subject to limitations depending on the magnitude of a rated primary current. For example, where the rated primary current has a relatively small magnitude, for example, 2,000 amperes max., the prior art current transformer device has an upright form, while, when the rated primary current is relatively large, such as 3,000 amperes min., the current transformer device assumes a'fallen fonn. The term upright form, as used herein, is defined to mean the type of current transformer device wherein the current transformer itself is provided with an iron core and the secondary winding thereof is disposed at the lower part of the current transformer device and the primary conductor is made to extend from the upper part of the current transformer device down to the ,site of the current transformer itself and then rise through the current transformer back to its starting position at the top of the device. The term fallen-form, as used herein, is defined to mean the type of current transformer device wherein the transformer itself is placed at the upper part or head of the current transformer device and the primary conductor linearly penetrates the core of the current transformer.

The conventional upright type current transformer device requires, as described above, the primary conductor to be first bent downward and then extended upward back to its starting point and consequently has the drawback that it not only presents diff culties in assembly, but also consumes an increased amount of copper from the primary conductor, though it offers an advantage from the standpoint of earthquake resistance. On the other hand, the fallen type cur,- rent transformer device allows the primary conductor to be used in linear form without being bent and has the advantage of reducing copper requirements for the primary conductor, but is undesirable from the quakeproof point of view. A need therefore, existed for a current transformer assembly which could be built in upright form and effectively operate even in cases where there is to be applied a large rated primary current.

Furthermore, such a current transformer having a fixed rated capacity, with circuits having various current ratings, would offer a great advantage in instituting standardization in the manufacture of current transformer assemblies.

, SUMMARY or THE INVENTION Briefly, in accordance with this invention the foregoing and other objects are attained by the provision of a current transformer assembly which includes a main circuit having a shunt circuit consisting of n (n a 2) number of shunt conductors; a

main current transformer for transforming current passing through one of the shunt conductors; and n 1 number of current balancers connected to the main circuit. The secondary windings of paired subcurrent transformers which 'con-. stitute respective current balancers are connected to each 1 other to provide a circulating secondary current flow so that the balancers will jointly act to fix the ratio of the magnitude of the currents introduced through the shunt conductors.

Since the magnitude of the current flowing through the conductors constituting the shunt circuit are maintained in a prescribed ratio by the action of the current transformers, only one of the shunt conductors is required to be connected to the main current transformer. This resolves the problem encountered with the prior art current transformer device wherein if a large current were to be introduced through the main conductor of the main circuit, difficulties would occur in constructing the device in an upright form. Moreover, the fact that a main current transformer having a certain fixed rated capacity can be used with respect to various ratings of current passing through the main conductor simply by changing a shunt circuit, permits the standardized manufacture of a current transformer device.

The subcurrent transformers may be connected only to the shunt circuit or both the main conductor and the shunt circuit.

Furthermore, where the secondary windings of the subcurrent BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 isa longitudinal sectional view of an upright type current transformer assembly according to an embodiment of thisinvention; a

FIG. 2A is an electric circuit diagram of FIG. 1;

FIG. 2B is a sectional view of a current balancer included in the embodiment of FIG. 1;

FIG. 3 is an electric circuit diagram showing the relationship of the main current transformer and the subcurrent transformers where three conductors are used in the shunt circuit of FIG. 1;

FIG. 4 is a longitudinal sectional view of an upright type current transformer assembly according to another embodiment of the present invention;

FIG. 5 is an electric circuit of FIG. 4;

FIG. 6 isan electric circuit showing the interconnection of the main current transformer and the subcurrent transformers where for the case of three shunt conductors are used in FIG. 4; and

FIGs. 7 and 8 respectively are circuit diagrams showing the interconnection of the shunt conductors and subcurrent transformers of the embodiments of the invention when provided with switch taps.

DETAILED DESCRIPTION GF THE PREFERRED EMBODIMENTS Referring now to the drawings wherein like reference numerals designate identical, or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof wherein the main circuit is shown as including a main conductor 1 connected to a source (not shown), a shunt circuit 2, having shunt conductors 2a and 2b and another main conductor 3, connected to a load. A current balancer 4a is provided across the shunts to fix the ratio of the magnitude of the currents passing through the shunt conductors 2a and 211. An intermediate part 15 of the shunt conductor 2b extends downward and penetrates an annular core 5a about which is wound the secondary winding 6 of a main current transformer 5 to thereby constitute its primary winding. The shunt circuit 2 is integrally formed with the current balancer 4d and received in a casing 7. The main conductors 1 and 3 are drawn out through insulation bushings 8. The main current transformer 5 is enclosed in a casing 9 and leads 10 of the secondary winding are drawn outside of the casing 9. A ring-shaped bushing support 13 is fixed to the upper cap of the casing 9. An insulation bushing 14 is fitted in a conventional manner to the inner wall of the ring support 13. The balancer casing 7 is fixed to the upper end of the insulation bushing 14. The aforementioned intermediate conductor 15 of the shunt conductor 2b penetrates an insulation material 16 disposed in the bushing 14. The outer wall of the insulation material l6'is surrounded by a condenser formed of several layers of aluminum foil 12 thereby enabling a uniform potential gradient from the intermediate conductor 15 to the outside to be realized. Both casings 7 and 9 are filled with an insulation oil or gas. These insulation fluids communicate with each other through a passage 17 formed in the bushing 14. I

As shown in FIG. 2B, the current balancer 4a includes a subcurrent transformer 18a which is penetrated by the shunt conductor 2a acting as a primary conductor and a subcurrent transformer 18b which is penetrated by the shunt conductor 2b acting similarly as a primary conductor. The secondary windings 19a and 19b of the subcurrent transformers 18a and 18b are connected to cause a circulating secondary current to flow to each other, that is, the different polarities thereof are connected. It should be understood that the circuit of the current transformer assembly of FIG. 1 may be schematically indicated by an equivalent circuit given in FIG. 2A. In this figure, I denotes the total current passing through themain conductor 1, I the current of the shunt conductor'2a, I the current of the shunt conductor 212, I a circulating current through the secondary windings 19a and 19b, and 20 a load to the main current transformer 5.

The aforementioned current transformer assembly of this invention enables the shunt currents I and I to be maintained in a prescribed ratio by the action of the current balancer 4a and the main current transformer 5 will carry out transformation in a fixed ratio, so that the total current I of the main conductor 1 can be accurately measured.

With the ratio of the shunt current I to the shunt current I being represented by K and the ratio of transformation conducted by the main current transformer 5 beingrepresented by K then the ratio K of transformation effected by the current transformer assembly of the present invention may be expressed by the following equation:

Now let the number of turns of the primary and secondary coils of the subcurrent transformer 18a be designated as T and T the number of turns of the primary and secondary coils of the subcurrent transformer 18b as T and T the ratio of transformation carried out by the subcurrent transformer 18a as K, T /I and the ratio of transformation by the subcurrent transformer 18b as K, T ff Now when a circulating current I is conducted through the current balancer 4a,the excitation ampere-tums AT,, and AT remaining in the cores of the respective subcurrent transformers 18a and 18b may be expressed as In general, with a current transformer AT, and AT,, are

negligibly small and as such the following equation results:

I,-T =I -T 3.

z sr c B2 When the ratio of I to I in establishing the above equation (3) is denoted by K and I and I are supposed to have a relationship ofI I K it will be apparent that AT will become positive and AT negative. The positive ampere-tums AT will generate a voltage acting to decrease the shunt current I; in the primary coil (having turns T of the subcurrent transformer 18a and the negative ampere-turns AT will produce a voltage acting to increase the shunt current I, in the primary coil (h'aving turns T of the subcurrent transformer 18b.

Since regulation of the shunt current is automatically carried out, the ratio of the shunt currents, i.e., l ll K, attained at the final equalization may be given as I a conductor 1 is required to be supplied to the main current transformer 5, it is easy to construct the current transformer assembly of the present invention in an upright form and also to reduce temperature rises in the main current transformer 5. Further, a main current transformer of the same construction can be used with various magnitudes of current simply by changing the shunt circuit and current balancer according to the current of the main conductor thus making it possible to standardize the form of the main current transformer.

When a far longer rated current is to be introduced through the main conductor 1, the current transformer assembly of the present invention can eflect transformation in a still larger current transformation ratio simply by using an increased number of shunt conductors. FIG. 3 illustrates an electric circuit corresponding to FIG. 2A, but including shunt conductors 2a, 2b and 20. In the case of FIG. 3, a current balancer 4b is provided between the shunt conductors 2a and 2b and another current balancer 4c is provided between the shunt conductors 2b and 2c. The shunt conductor 20 is connected to the main transformer 5. The current balancers are of an arrangement similar to that of FIG. 2A and are thus designed to stabilize the current passing through the two shunt conductors associated therewith. Now let the ratio of current transformation conducted by subcurrent transformers 18c, 18d, 18c and 18f be represented by K K K and K and chosen so as to have a relationship of K, K K}, K Then it will be apparent that the ratio of the currents 1,, I, and I passing through the shunt conductors 2a, 2b and 20. In the case where many shunt conductors are used, as described above, it'will be apparent that in actual operation it is possible to change the number n of shunt conductors to be used by arranging an arbitrary number thereof so as to be detachably fitted to, for example a known blade.

It should be understood that the current balancers need not always be connected only to a shunt conductor, but may be coupled, as shown in FIG. 4, with the shunt conductor and main conductor with a prescribed relationship maintained therebetween. According to the embodiment of FIG. 4, a sub- 2b and another subcurrent transformer 18h is connected to the main conductor 1. The respective secondary windings 19g and 19b of the subcurrent transformers 18g and 18h are connected to cause a circulating secondary current to flow to each other as illustrated in FIG. 5. That is, the paired subcurrent transfon'ners 18g and 18h jointly constitute a current balancer.

Now let the current of the main conductor 1 be represented by l and the currents of the shunt conductors 2a and 2b respectively. 1 and i as shown in FIG. 5, the ratio of i n, by K and the ratio of transformation by the main current transformer 5 by K Then the ratio of transformation by the current transformer assembly as a whole may be given as K K K 5.

Now let the number of turns of the primary coil of the subcurrent transformer 18h be denoted by T the number of turns of the-secondary coil thereof by T the ratio of transformation by the subcurrent transformer 18h by K =(T /T the number of turns of the primary coil of the other subcurrent transformer-18g by T the number of turns of the secondary coil thereof by T and the ratio of transformation by the other current transformer 18g by K, =(T /l Now when a circulating secondary current I is introduced excitation amperetums AT, and AT will be left in the cores of the subcurrent transformersl8h and 18g and be expressed by the equatio 6) below.

In general with a current transformer. the ampere-tums are negligibly small, so that the following equation results:

. 'I T I T 7.

Where, therefore, l and I have a relationship 1 I K (where K represents the ratio of 1 to 1 when the equation (7) above is established), then the ampere-turns AT will become positive and the ampere-turns AT negative. The positive ampere-tums AT will generate a voltage acting to decrease the current I of the main conductor 1 in the primary coil (having turns T of the subcurrent transfonner 18h and the negative ampere-tums AT will generate a voltage acting to increase the shunt current I, in the primary coil (having turns T of the subcurrent transformer 18g. Thus'the ratio of 1 /1 is controlled to have a fixed value. The ratio X (I /I of transformation is expressed as Therefore, the ratio K of transformation by the current transformer assembly of FIG. 4 may be determined by substituting the value of the equation (8) in equation (5). It should be understood that the current transformer assembly of FIG. 4 will attain the same objects as that of the embodiment of FIG. 1.

Furthermore, it should be apparent that the present invention will enable a much larger rated current to be measured by a main current transformer having a fixed rated capacity simply by adding more shunt conductors. When shunt conductors 2a, 2b and 21:, as shown in FIG. 6, are used it is necessary to connect only subcurrent transformers l8i and 18g to the main conductor 1, a subcurrent transformer 181 to the shunt conductor 2b and a subcurrent transformer 18k to the shunt conductor 20 and to connect the two secondary windings of the. subcurrent transformers 181 and 18k and to connect the been found that sometimes a slight error occurs in the ratio of the shunted current. In order to avoid such errors, it is preferred that the secondary windings of the subcurrent transformers each be provided with a plurality of switch taps t, to

t as shown in FIGS. 7 and 8. By the action of such switch which are separate from the main current transformers.

It is there ore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and intended to be secured by letters patent of the United States is: I

two secondary windings of the subcurrent transformers 19g and 181 to cause a circulating current to flow. It will be apparent that proper selection of the ratio of transformation by 1. A current transformer assembly comprising:

a main circuit including a shunt circuit having n number (n E 2) of shunt conductors;

a main current transformer for transformating current passing through one of the shunt conductors included in said shunt circuit; and

(n-l) number of current balancers connected to the main circuit each of which is formed of a pair of subcurrent transformers and wherein the secondary windings of each pair of subcurrent transformers are connected to each other with different polarities through terminals on each of said secondary windings so as to cause a circulating current flow such that the balancers will jointly act to fix the magnitude of the currents passing through the shunt conductors.

2. A current transformer assembly according to claim 1,

wherein the subcurrent transformers constituting said (n-l) number of current balancers are connected to the shunt circuit.

3. A current transformer assembly according to claim 1, wherein the (n-I numberof current balancers comprises:

(n-l number of first subcurrent transformers connected to a main conductor through which is introduced the total current of the shunt circuit:

(n-l) number of second subcurrent transformers connected to the shunt circuit; and wherein each of said balancers consists of one first subcurrent transformer and one second subcurrent transformer. 4. A current transformer assembly according to claim I, wherein the secondary windings of the current transformers constituting the current balancers are provided with a plurality of switch taps so as to vary the ratio of the magnitude of the currents passing through the shunt conductors.

5. A current transformer assembly according to claim 1, v 

1. A current transformer assembly comprising: a main circuit including a shunt circuit having n number (n > OR = 2) of shunt conductors; a main current transformer for transformating current passing through one of the shunt conductors included in said shunt circuit; and (n-1) number of current balancers connected to the main circuit each of which is formed of a pair of subcurrent transformers and wherein the secondary windings of each pair of subcurrent transformers are connected to each other with different polarities through terminals on each of said secondary windings so as to cause a circulating current flow such that the balancers will jointly act to fix the magnitude of the currents passing through the shunt conductors.
 2. A current transformer assembly according to claim 1, wherein the subcurrent transformers constituting said (n- 1) number of current balancers are connected to the shunt circuit.
 3. A current trAnsformer assembly according to claim 1, wherein the (n- 1) number of current balancers comprises: (n- 1) number of first subcurrent transformers connected to a main conductor through which is introduced the total current of the shunt circuit: (n- 1) number of second subcurrent transformers connected to the shunt circuit; and wherein each of said balancers consists of one first subcurrent transformer and one second subcurrent transformer.
 4. A current transformer assembly according to claim 1, wherein the secondary windings of the current transformers constituting the current balancers are provided with a plurality of switch taps so as to vary the ratio of the magnitude of the currents passing through the shunt conductors.
 5. A current transformer assembly according to claim 1, wherein the main current transformer and current balancers are respectively received in a main current transformer casing and a balancer casing each of which are filled with an insulation fluid, and wherein the balancer casing is supported above the main current transformer casing through a bushing provided with a passage for the insulation fluid and is penetrated by the primary conductor of the main current transformer. 